High-voltage pulse generating circuit

ABSTRACT

A high-voltage pulse generating circuit has an inductor, a first semiconductor switch, and a second semiconductor switch which are connected in series between opposite terminals of a DC power supply unit, and a diode having a cathode terminal connected to a terminal of the inductor which has another terminal connected to an anode terminal of the first semiconductor switch, and an anode terminal connected to a gate terminal of the first semiconductor switch. The inductor stores an induction energy when the first semiconductor switch is rendered conductive by a turn-on of the second semiconductor switch, and generates a high-voltage pulse when the first semiconductor switch is turned off by a turn-off of the second semiconductor switch.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.10/457,164, filed Jun. 9, 2003, now U.S. Pat. No. 7,084,528, theentirety of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high-voltage pulse generating circuitof a simple arrangement for supplying a high-voltage pulse having anextremely short rise time and an extremely short pulse duration byreleasing electromagnetic energy which has been stored in an inductorfrom a low-voltage DC power supply unit.

2. Description of the Related Art

There has recently been proposed a technology for generating a plasma bydischarging a high-voltage pulse to perform deodorization, sterilizationand also to decompose toxic gases. Generating such a plasma requires ahigh-voltage pulse generating circuit which is capable of generating apulse having a high voltage and an extremely short pulse duration.

As shown in FIG. 15 of the accompanying drawings, a conventionalhigh-voltage pulse generating circuit 100 comprises a capacitor charger102, a capacitor 104, a switch 108, and a load 110 (see JapaneseLaid-Open Patent Publication No. 2002-44965, for example).

The capacitor charger 102 generates a high DC voltage which issubstantially equal to the peak value of a high-voltage pulse. Thecapacitor 104 is charged by the capacitor charger 102 to a voltage whichis substantially equal to the high DC voltage generated by the capacitorcharger 102. In order for the switch 108 to have a large withstandvoltage, the switch 108 comprises a plurality of semiconductor devices106 such as SI (Static Induction) thyristors or the like which areconnected in series. The load 110 is supplied with a high-voltage pulseby high-speed switching operation of the switch 108 under the high DCvoltage charged in the capacitor 104.

The switch 108 has a plurality of gate drive circuits 112 connected tothe respective semiconductor devices 106 to turn on the semiconductordevices 106, and a plurality of balancing resistors 114 connectedparallel to the respective semiconductor devices 106. The balancingresistors 114 serve to reduce any unbalances between the voltagesapplied across the respective semiconductor devices 106 due to impedancevariations caused when the semiconductor devices 106 are renderednonconductive.

Specifically, the high-voltage pulse generating circuit 100 has amultiple-series-connected circuit 116 of semiconductor devices 106 andbalancing resistors 114 which are connected in series to the load 110.

FIG. 16 of the accompanying drawings shows a proposed high-voltage pulsegenerating circuit 118. In the proposed high-voltage pulse generatingcircuit 118, when a semiconductor switch 126 is turned on, a currentflows from a DC power supply 120 (having a power supply voltage E) to aresistor 136 (having a resistance R) to the one-turn primary windings ofrespective maginetizable cores 128 to the semiconductor switch 126 tothe DC power supply 120, the current having a magnitude representedsubstantially by E/R.

At this time, because of the maginetizable cores 128 operating as atransformer, the same current flows through the one-turn secondarywindings of respective maginetizable cores 128 via the gates andcathodes of semiconductor devices 134. Therefore, all the semiconductordevices 134 are simultaneously turned on (see, for example, TheInstitute of Electrical Engineers of Japan, Plasma Science andTechnology, Lecture No. PST-02-16).

The semiconductor devices 134 connected in series, and the semiconductorswitch 126 are rendered conductive, a voltage which is substantially thesame as the power supply voltage E is applied to an inductor 138. As aresult, a current I_(L) flowing through the inductor 138 increaseslinearly, storing electromagnetic energy in the inductor 138.

The current I_(L) flowing through the inductor 138 increases untilelectromagnetic energy is stored up to a desired level in the inductor138. When the semiconductor switch 126 is turned off, since the path ofthe current I_(L) flowing through the inductor 138 is cut off, aninduced voltage of opposite polarity is generated due to the storedelectromagnetic energy in the inductor 138.

As a consequence, the diode 140 is rendered conductive, allowing acurrent to flow continuously from the inductor 138 to the semiconductordevices 134, the primary windings of the respective maginetizable cores128 to the diode 140 to the inductor 138. At this time, a current of thesame magnitude also flows through the secondary windings of themaginetizable cores 128.

Thus, the current flowing into the anodes of the semiconductor devices134 flows entirely into the gates thereof, with no current flowing tothe cathodes thereof. The current flows until the electric chargesstored in the semiconductor devices 134 are discharged. Since no largevoltage drop is caused in the current path and this state merelycontinues for an extremely short period of time, any reduction in thecurrent I_(L) flowing through the inductor 138 is small, and anyreduction in the stored electromagnetic energy in the inductor 138 isalso small.

As the electric charges stored in the semiconductor devices 134 aredischarged, the semiconductor devices 134 are turned off, with adepletion layer being quickly developed therein. Since the inductorcurrent is charged with a small electric capacity, the voltage betweenthe anode and cathode of each of the semiconductor devices 134 risessharply. Therefore, the voltage across the inductor 138 increasesquickly, and the current I_(L) flowing through the inductor 138decreases quickly. Stated otherwise, the electromagnetic energy in theinductor 138 is shifted into a capacitive electrostatic energy storedbetween the anode and cathode of each of the semiconductor devices 134.Since the voltage across the inductor 138 is also applied to a load 142connected across the inductor 138, the electromagnetic energy in theinductor 138 and the capacitive electrostatic energy stored between theanode and cathode of each of the semiconductor devices 134 are consumedby the load 142 while the electromagnetic energy is being shifted intothe electrostatic energy.

With the high-voltage pulse generating circuit 118, the DC power supply120 may generate a low voltage and the semiconductor devices 134 may beturned on and off only by currents flowing through the secondarywindings of the maginetizable cores 128. Consequently, the high-voltagepulse generating circuit 118 requires no gate drive circuits and isrelatively simple.

However, the conventional high-voltage pulse generating circuit 100shown in FIG. 15 has a complex circuit arrangement. A high voltage isapplied to all the circuit components including the capacitor charger102. The circuit components need to be insulated against each other,e.g., need to be spaced from each other by a large distance. Therefore,the conventional high-voltage pulse generating circuit 100 tends to belarge in size and high in cost.

If only some of the series-connected semiconductor devices 106 areturned on due to malfunctions, then the remaining semiconductor devices106 may be damaged by an overvoltage in excess of a rated voltageapplied thereto. Accordingly, the operation of the conventionalhigh-voltage pulse generating circuit 100 is not reliable.

Furthermore, for the conventional high-voltage pulse generating circuit100 to generate a pulse which rises extremely sharply, e.g., at 10kV/μsec or above, it is necessary that each of the semiconductor devices106 be turned on quickly. Consequently, even if gate signals are appliedto the semiconductor devices 106 at timings differing merely by 2 nsecor 3 nsec, or semiconductor devices 106 are turned on at timingsdiffering merely by 2 nsec or 3 nsec, generated transient voltages areliable to be out of balance. The conventional high-voltage pulsegenerating circuit 100 thus suffers greater difficulty generating apulse at several hundreds V/μsec than a series-connected array ofsemiconductor devices in an ordinary inverter.

With the proposed high-voltage pulse generating circuit 118 shown inFIG. 16, however, the DC power supply 120 may generate a low voltage,and a voltage in excess of the withstand voltage is never be applied tothe semiconductor devices 134 even if some are turned off due tomalfunctions. However, the timings of the turning off of thesemiconductor devices 134 differ, making it highly difficult to preventtransient voltages from being brought out of balance when thesemiconductor devices 134 are turned off quickly. Therefore, theproposed high-voltage pulse generating circuit 118 also suffers the sameproblems of series-connected semiconductor devices.

In the high-voltage pulse generating circuit 118, the maginetizablecores 128 are connected in series to the diode 140. As a consequence,inductances exist due to the physical distance in which themaginetizable cores are provided and also due to leakages between thefinite primary and secondary windings. Because of these inductances, ittakes time for the inductor current, which flows when the semiconductorswitch 126 is turned off, to be commutated to the diode 140. As aresult, the rate at which the gate current increases is suppressed,causing the semiconductor device 134 to remain conductive longer and thedepletion region to spread (with the turn-off gain becoming one ormore), which makes the high-voltage pulse generating circuit 118unstable when the semiconductor devices 134 are turned off sharply.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide ahigh-voltage pulse generating circuit which is of a simple arrangementfree of a plurality of semiconductor switches to which a high voltage isapplied, and is capable of supplying a high-voltage pulse having anextremely short rise time and an extremely short pulse duration.

A high-voltage pulse generating circuit according to the presentinvention has an inductor, a first semiconductor switch, whichpreferably has as high a voltage rating as possible, and a secondsemiconductor switch, which may have a voltage rating as low as a DCpower supply voltage, connected in series between opposite terminals ofa DC power supply unit. A diode has a cathode terminal connected to aterminal of the inductor whose other terminal is connected to an anodeterminal of the first semiconductor switch, and an anode terminalconnected to a control (gate) terminal of the first semiconductorswitch. The high-voltage pulse generating circuit thus has a highlysimple arrangement.

When the second semiconductor switch is turned on, the firstsemiconductor switch is rendered conductive, applying the voltage of theDC power supply unit to the inductor, which stores an induction energytherein. When the second semiconductor switch is subsequently turnedoff, the first semiconductor switch is also turned off quickly.Therefore, the inductor generates a high-voltage pulse having anextremely short rise time and an extremely short pulse duration.

A load which is supplied with the high-voltage pulse from the inductormay be connected parallel to the inductor or parallel to the firstsemiconductor switch.

According to the present invention, the inductor may comprise a primarywinding and a secondary winding magnetically coupled to each other.Since a voltage which is substantially the same as the voltage generatedby the inductor is applied to the first semiconductor switch, thevoltage generated by the inductor cannot be set to a level equal to orhigher than the withstand voltage of the first semiconductor switch.

If the high-voltage pulse generating circuit is required to produce anoutput voltage higher than the withstand voltage of the firstsemiconductor switch, then the secondary winding may have a greaternumber of turns than the primary winding to generate a high-voltagepulse across the secondary winding of the inductor, which has a voltagehigher than the withstand voltage of the first semiconductor switch.

The inductor may comprise a primary winding and a secondary windingconnected to a primary winding without insulating a DC signal. Thesecondary winding may be connected to a primary winding withoutinsulating a DC signal, but connected to the primary winding in additivepolarity, for outputting a high-voltage pulse having a voltage which isthe sum of voltages generated across the primary and secondary windings.

If the inductor is constructed of the primary and secondary windings,the inductor should preferably have a maginetizable core to provide aclose magnetic coupling between the primary and secondary windings andsuppress a magnetic flux leakage therefrom.

Devices which can be used as the first semiconductor switch will bedescribed below. The first semiconductor switch may comprise a devicecontrolled based on current or a self-extinguishing orcommutation-turn-off device. Specifically, the first semiconductorswitch may comprise an SI thyristor, a GTO (Gate-Turn-Off) thyristor, anSIT (Static Induction Transistor), a bipolar transistor, a thyristor, orthe like. Of these devices, a current-controlled, self-extinguishingthyristor such as a GTO is preferable. Particularly, if a device havinghigh turn-on and turn-off speeds is required for generating ashort-duration pulse, then an SI thyristor is preferable for use as thefirst semiconductor switch. A field effect alone can turn on an SIthyristor by applying a slight positive voltage between the gate andcathode thereof if the current rise rate at the time it is turned on isrelatively small.

When the first semiconductor switch is to be turned off, a current isdrawn from the gate thereof to eliminate electric charges stored in thefirst semiconductor switch, developing a depletion layer therein tocompletely turn off the first semiconductor switch. If the high-voltagepulse generating circuit is used in an ordinary inverter or the like,then a turn-off of the first semiconductor switch can be achieved whenthe drawing of the current from the gate is completed even if theturn-off gain is equal to or greater than 1, i.e., even if the gatecurrent is smaller than the anode current and does not have a highincrease rate.

If a sharp turn-off needs to be achieved such as in a pulse powerapplication, then it is necessary to make the turn-off gate currentequal to the anode current (the turn-off gain is 1) or greater than theanode current (the turn-off gain is less than 1) and quickly increasethe turn-off gate current, and to achieve an ideal stable turn-off toeliminate the cathode current before the drawing of the electric chargesstored in the first semiconductor switch is completed.

However, since the anode current is usually high, it is very difficultand not practical for an ordinarily gate drive circuit to supply suchcurrent from the gate in order to make the turn-off gain equal to orless than 1 and to turn off the first semiconductor switch sharply (in atime of ten and several nsec. until the gate current is equalized to theanode current).

Functionally, the high-voltage pulse generating circuit according to thepresent invention makes the turn-off gain equal to or less than 1apparently without the need for such a gate drive circuit.

The second semiconductor switch may comprise a self-extinguishing orcommutation-turn-off device. For example, the second semiconductorswitch may comprise a power metal-oxide semiconductor field-effecttransistor.

The high-voltage pulse generating circuit may further comprise a circuitcomponent connected to regenerate remaining energy in the inductor inthe DC power supply unit after the second semiconductor switch is turnedoff.

The above circuit component may comprise a diode connected parallel tothe first semiconductor switch and having a cathode terminal connectedto the anode terminal of the first semiconductor switch. Alternatively,the circuit component may comprise a diode having an anode terminalconnected between the DC power supply unit and the second semiconductorswitch, and a cathode terminal connected to the other terminal of theinductor.

With the above arrangement, if any energy remains in the inductor, e.g.,if a load is connected to the inductor, then excessive energy (unusedenergy) from the load is returned to the DC power supply unit,contributing to a higher efficiency of operation of the DC power supplyunit.

The high-voltage pulse generating circuit may further comprise a pathfor commutating a current flowing through the first semiconductor switchafter the second semiconductor switch is turned off. The path may beconnected parallel to the first semiconductor switch.

The path may have a capacitor connected between the anode and cathodeterminals of the first semiconductor switch. Alternatively, the path mayhave a capacitor connected between the gate and anode terminals of thefirst semiconductor switch. The path with the capacitor reduces theoperational burden of the first semiconductor switch. The path with thecapacitor is effective to reduce the switching loss caused by the firstsemiconductor switch and to increase the current cutoff resistance ofthe first semiconductor switch.

When the first semiconductor switch cuts off a current at a high speedor cuts off a large current, a large surge voltage is applied to theexcited inductance of the inductor and the first semiconductor switch.However, the above path is effective to reduce the surge voltage forthereby increasing the reliability of the first semiconductor switch.

The first semiconductor switch which is used may not have a high voltagerise rate (dv/dt) at the time it is turned off. The above path with thecapacitor is effective to adjust the voltage rise rate (dv/dt) of thefirst semiconductor switch to an allowable level with the capacitance ofthe capacitor.

Since much of the energy remaining in the capacitor thus connected isregenerated in the DC power supply unit, any reduction in the efficiencywhich is caused by the capacitor is small.

If a load is connected to the inductor, then a capacitor may beconnected parallel to the load. The capacitor thus connected makes iteasy for the excited inductance of the inductor to commutate a currentto the load after the first semiconductor switch cuts off the current.As with the path described above, the capacitor thus connected is alsoeffective to reduce the switching loss caused by the first semiconductorswitch and increase the current cutoff resistance of the firstsemiconductor switch. The capacitor connected parallel to the load canabsorb the energy stored in the excited inductance of the inductor, thussuppressing the surge voltage on the excited inductance. Because much ofthe energy remaining in the capacitor thus connected is also regeneratedin the DC power supply unit, any reduction in the efficiency which iscaused by the capacitor is small.

According to the present invention, another high-voltage pulsegenerating circuit comprises a DC power supply unit having oppositeterminals, an inductor, a first semiconductor switch, and a secondsemiconductor switch which are connected in series between the oppositeterminals of the DC power supply unit, and a resistor connected betweena terminal of the inductor which has another terminal connected to ananode terminal of the first semiconductor switch, and a gate terminalconnected to a gate terminal of the first semiconductor switch.

With the above arrangement, when the second semiconductor switch isturned on, the first semiconductor switch is reliably turned on. If thefirst semiconductor switch comprises is controlled based on current,then it is not turned on unless a current is introduced into the gatethereof. The resistor connected as described above is effective inreliably turning on the first semiconductor switch.

Use of the resistor makes the high-voltage pulse generating circuitrelatively low in cost even if the DC power supply unit is constructedto produce a high power supply voltage.

The above and other objects, features, and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which preferredembodiments of the present invention are shown by way of illustrativeexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a high-voltage pulse generating circuitaccording to a first embodiment of the present invention;

FIGS. 2A through 2E are waveform diagrams showing the waveforms ofvoltages and currents in the high-voltage pulse generating circuitaccording to the first embodiment;

FIG. 3 is a circuit diagram of a high-voltage pulse generating circuitaccording to a second embodiment of the present invention;

FIG. 4 is a circuit diagram of a high-voltage pulse generating circuitaccording to a third embodiment of the present invention;

FIG. 5 is a circuit diagram of a high-voltage pulse generating circuitaccording to a fourth embodiment of the present invention;

FIG. 6 is a circuit diagram of the high-voltage pulse generating circuitaccording to the fourth embodiment, the view showing the manner in whicha current flowing through the excited inductance of an inductor flows toa load through the inductor;

FIG. 7 is a circuit diagram of the high-voltage pulse generating circuitaccording to the fourth embodiment, the view showing the manner in whichenergy is regenerated;

FIG. 8 is a circuit diagram of a high-voltage pulse generating circuitaccording to a fifth embodiment of the present invention;

FIG. 9 is a circuit diagram of a high-voltage pulse generating circuitaccording to a sixth embodiment of the present invention;

FIG. 10 is a circuit diagram of a high-voltage pulse generating circuitaccording to a seventh embodiment of the present invention;

FIG. 11 is a circuit diagram showing the manner in which a currentflowing through a first semiconductor switch flows to a capacitor;

FIG. 12A is a diagram showing how the voltage between the anode andcathode of the first semiconductor switch differs when a capacitor isnot connected and when a capacitor is connected;

FIG. 12B is a diagram showing how a switching loss differs;

FIG. 13 is a circuit diagram of a high-voltage pulse generating circuitaccording to an eighth embodiment of the present invention;

FIG. 14 is a circuit diagram of a high-voltage pulse generating circuitaccording to a ninth embodiment of the present invention;

FIG. 15 is a circuit diagram of a conventional high-voltage pulsegenerating circuit; and

FIG. 16 is a circuit diagram of a proposed high-voltage pulse generatingcircuit.

DETAILED DESCRIPTION OF THE INVENTION

High-voltage pulse generating circuits according to various embodimentsof the present invention will be described below with reference to FIGS.1 through 14. Similar or corresponding parts are denoted by similar orcorresponding reference characters throughout views.

As shown in FIG. 1, a high-voltage pulse generating circuit 10Aaccording to a first embodiment of the present invention has an inductor32, a first semiconductor switch 34, and a second semiconductor switch14 which are connected in series between positive and negative terminals46, 48 of a DC power supply unit 12. The DC power supply unit 12comprises a DC power supply 22 for generating a DC power supply voltageE and a capacitor 24 for lowering a high-frequency impedance. Theinductor 32 has a terminal 44 connected to an anode terminal A of thefirst semiconductor switch 34 and another terminal 42 connected to acontrol terminal (gate terminal) G of the first semiconductor switch 34through a diode 36. The diode 36 has an anode connected to the controlterminal G of the first semiconductor switch 34. A load 20 to which ahigh-voltage pulse will be applied is connected parallel to the inductor32.

In the embodiment shown in FIG. 1, the second semiconductor switch 14 isconnected to the negative terminal 48 of the DC power supply unit 12.However, the second semiconductor switch 14 may alternatively beconnected to the positive terminal 46 of the DC power supply unit 12.Although the load 20 is connected parallel to the inductor 32 in FIG. 1,the load 20 may be connected parallel to the first semiconductor switch34.

The second semiconductor switch 14 may comprise a self-extinguishing orcommutation-turn-off device. In the first embodiment, the secondsemiconductor switch 14 comprises a power metal-oxide semiconductorfield-effect transistor (power MOSFET) 26 with an avalanche diode 30 ininverse-parallel connection. The second semiconductor switch 14 also hasa gate drive circuit 28 connected to a gate terminal G and a sourceterminal S of the power MOSFET 26 for controlling the turning on and offof the power MOSFET 26.

The first semiconductor switch 34 may comprise a device controlled bycurrent, or a self-extinguishing or commutation-turn-off device. In thefirst embodiment, the first semiconductor switch 34 comprises an SIthyristor having a very large resistance with respect to a voltage riserate (dv/dt) at the time it is turned off and also having a high voltagerating.

Operation of the high-voltage pulse generating circuit 10A according tothe first embodiment will be described below primarily in achronological sequence for supplying a high-voltage pulse V_(L) to theload 20 with reference to the circuit diagram shown in FIG. 1 andwaveform diagrams shown in FIGS. 2A through 2E.

At time t₀, the gate drive circuit 28 supplies a control signal Vc (seeFIG. 2E) between the gate and source of the power MOSFET 26, which isturned on from an off state.

At this time, the first semiconductor switch 34 is turned on by a fieldeffect caused by applying a positive voltage between the gate G andcathode K thereof (see FIG. 2D) because of a very large impedanceprovided in opposite polarity by the diode 36. The first semiconductorswitch 34 is normally turned on only due to the field effect since theanode current of the first semiconductor switch 34 is prevented fromrising by the inductor 32. Alternatively, a resistor may be connectedparallel to the diode 36 or a resistor may be connected from anotherpower supply to the gate of the first semiconductor switch 34, and alarge gate current may be supplied to the gate terminal G of the firstsemiconductor switch 34 through such a resistor.

When the second semiconductor switch 14 and the first semiconductorswitch 34 are thus rendered conductive at time t₀, a voltage, which issubstantially the same as the DC power supply voltage E, is applied tothe inductor 32. If the inductance of the inductor 32 is represented byL, then as shown in FIG. 2A, a current I_(L) flowing through theinductor 32 increases linearly with time at a gradient represented byE/L.

When the current I_(L) reaches a level Ip (=ET₀/L) at time t₁, storing adesired amount of electromagnetic energy (=LIp²/2) in the inductor 32,the gate drive circuit 28 stops supplying the control signal, turningoff the power MOSFET 26 (see FIG. 2E).

At this time, if a floating inductance (mainly a wiring inductance), notshown, other than the inductance of the inductor 32, in the path of thecurrent I_(L) is large, then the power MOSFET 26 is not cut offinstantaneously. Specifically, the current continues to flow for a shortperiod of time, and the output capacitance of the power MOSFET 26 ischarged up to the avalanche voltage of the diode 30, whereupon the diode30 is rendered conductive under the avalanche voltage and hence suffersserious damage. To avoid the above drawback, the floating inductance isideally minimized allowing the power MOSFET 26 to be turned off withoutcausing an avalanche across the diode 30.

When the power MOSFET 26 is turned off, the current from the cathode Kof the first semiconductor switch 34 is eliminated, i.e., the firstsemiconductor switch 34 is opened. Therefore, the current I_(L) flowingthrough the inductor 32 is cut off, and the inductor 32 generates areverse induced voltage V_(L) due to the remaining electromagneticenergy stored therein. At this time, however, the diode 36 operates tocommutate the current I_(L) flowing through the inductor 32 to the pathfrom the anode terminal A of the first semiconductor switch 34 to thegate terminal G of the first semiconductor switch 34 to the anode of thediode 36 to the cathode of the diode 36.

It is necessary that any floating inductance of a branch circuitincluding the diode 36 be as small as possible to finish the commutationof the current within a short period of time. Any voltage drop acrossdiode 36 and inductor 32 is small because the first semiconductor switch34 remains conductive between the anode and gate until the storedelectric charges from the current that has already flowed becomes nil (astorage period).

Therefore, the reverse induced voltage V_(L) across the inductor 32 issuppressed to a sufficiently low value, that there is almost noreduction in the current I_(L) in the short storage period (i.e., aperiod T₁ in FIG. 2A). The period T₁ is determined based on the amountof electric charges drawn from the gate terminal G of the firstsemiconductor switch 34. To shorten the period T₁ and minimize anyreduction in the current I_(L) through the inductor 32, the firstembodiment of the present invention sharply passes as large a current aspossible (which cannot be larger than the anode current in the firstembodiment) to set the apparent turn-off gain to 1 or less.

At time t₂, the electric charges are completely drawn from the firstsemiconductor switch 34, and a depletion layer in the firstsemiconductor switch 34 spreads from the gate and the cathode toward theanode, starting to turn off the first semiconductor switch 34. Becausethe depletion layer depends on a potential developed in the firstsemiconductor switch 34, the depletion layer spreads as the voltageapplied to the junction increases and the turn-off process progresses,and finally reaches a position near the anode.

Therefore, the electric capacitance of the depletion layer changes froma saturated state (conductive state) where many active electric chargesare present, to a small electric capacitance which is structurallydetermined. The current continuously flows from the anode to the gate ofthe first semiconductor switch 34, thus charging the electriccapacitance of the depletion layer from the electromagnetic energystored in the inductor 32. The voltage for charging the electriccapacitance, i.e., the anode-to-gate voltage V_(AG) of the firstsemiconductor switch 34, initially increases relatively gradually due tothe large electric capacitance, but then increases quickly as thedepletion layer spreads.

When the current I_(L) becomes nil at time t₃, the voltage V_(AG) andthe voltage V_(L) reach maximum levels V_(AP), V_(LP), respectively, asshown in FIGS. 2B and 2C. At this time, the electromagnetic energystored in the inductor 32 has entirely been shifted into the electriccapacitance of the depletion layer in the first semiconductor switch 34.

This phenomenon is a resonant action based on the inductance of theinductor 32 and the electric capacitance of the first semiconductorswitch 34. Consequently, the current I_(L) flowing through the inductor32 is essentially of a cosine waveform, and the anode-to-gate voltageV_(AG) of the first semiconductor switch 34 is essentially a sinewaveform.

By selecting an appropriate inductance value for the inductor 32, theduration of a pulse generated across the inductor 32 and the load 20connected in parallel to the inductor 32 can be controlled.Specifically, if the electric capacitance of the first semiconductorswitch 34 is represented by an equivalent capacitance C, then the pulseduration Tp is expressed by:Tp≅π√{square root over (LC)}

The electric charges stored in the electric capacitance of the depletionlayer in the first semiconductor switch 34, which has been charged tothe maximum level V_(AP) at time t₃, start to be discharged through apath from the inductor 32 to the diode 36 which has been renderedreversely conductive by the stored electric charges. The electriccharges are continuously discharged for a period T₃ until the diode 36recovers itself and becomes nonconductive at time t₄. If any energyremains in the inductor 32 and the electric capacitance of the depletionlayer in the first semiconductor switch 34 at time t₄, then a currentdue to the remaining energy flows from the DC power supply unit 12 tothe diode 30 of the second semiconductor switch 14 to the cathode K ofthe first semiconductor switch 34 to the anode A of the firstsemiconductor switch 34.

During a period T₄ in which the current flows in the DC power supplyunit 12, the high-voltage pulse generating circuit 10A operates in aregenerative mode. In the regenerative mode, the energy which remains inthe inductor 32 and the electric capacitance of the depletion layer inthe first semiconductor switch 34 is regenerated and contributes to anincrease in the operating efficiency of the high-voltage pulsegenerating circuit 10A. Therefore, it is important to reduce the timerequired to recover the diode 36, i.e., the period T₃, as much aspossible.

In the first embodiment, the load 20 comprises a linear load which maybe an equivalent resistive load. If the load 20 comprises a nonlinearload such as a discharging gap 50 as shown in FIG. 5, then the loadimpedance is quickly reduced while the voltage is increases, andsubsequent waveforms are different from those shown in FIGS. 2B and 2C,i.e., subsequent waveforms are pulse-like waveforms whose pulsedurations are shorter than those shown in FIGS. 2B and 2C.

In the high-voltage pulse generating circuit 10A according to the firstembodiment shown in FIG. 1, the anode-to-gate voltage V_(AG) of thefirst semiconductor switch 34 is substantially the same as the voltageacross the inductor 32. Therefore, a voltage which is equal to or higherthan the resistance against the anode-to-gate voltage V_(AG) of thefirst semiconductor switch 34 cannot be outputted as a pulse from theinductor 32.

FIGS. 3 and 4 show high-voltage pulse generating circuits 10B, 10Caccording to second and third embodiments of the present invention. Thehigh-voltage pulse generating circuits 10B, 10C are suitable foroutputting a voltage which is equal to or higher than the resistanceagainst the anode-to-gate voltage V_(AG) of the first semiconductorswitch 34.

As shown in FIG. 3, the high-voltage pulse generating circuit 10Baccording to the second embodiment is substantially the same as thehigh-voltage pulse generating circuit 10A according to the firstembodiment, but differs in that the inductor 32 comprises a primarywinding 33 and a secondary winding 38, which is magnetically coupled tothe primary winding 33 and has a greater number of turns than theprimary winding 33.

As shown in FIG. 4, the high-voltage pulse generating circuit 10Caccording to the third embodiment is substantially the same as thehigh-voltage pulse generating circuit 10A according to the firstembodiment, but differs in that the inductor 32 comprises a primarywinding 33 and a secondary winding 38 connected to the primary winding33 without insulating a DC signal, but connected to the primary winding33 in additive polarity.

In the second and third embodiments, the primary and secondary windings33, 38 should preferably be wound around a maginetizable core to providea close magnetic coupling therebetween and suppress a magnetic fluxleakage therefrom.

If the number of turns of the primary winding 33 is represented by N1,the number of turns of the secondary winding 38 by N2 and the ratiobetween the numbers of turns by n(=N2/N1), then the high-voltage pulsegenerating circuit 10B according to the second embodiment can output avoltage of V_(AG)×N2/N1=V_(AG)×n to the load 20, and the high-voltagepulse generating circuit 10C according to the third embodiment canoutput a voltage of V_(AG)×(N1+N2)/N1 to the load 20.

In the second embodiment, the number of turns of the secondary winding38 is greater than the number of turns of the primary winding 33 suchthat the secondary winding 38 is of additive polarity. However, thenumber of turns of the secondary winding 38 may also be smaller than thenumber of turns of the primary winding 33 such that the secondarywinding 38 is of subtractive polarity.

In the third embodiment, the secondary winding 38 is not connected tothe primary winding 33 without insulating a DC signal, but connected tothe primary winding 33 in additive polarity. However, the secondarywinding 38 may also be wound as subtractive-polarity turns connected tothe primary winding 33.

To connect the secondary winding 38 in subtractive polarity to theprimary winding 33, the secondary winding 38 may be wound around amaginetizable core, for example, in a direction opposite to thedirection in which the secondary winding 38 is wound in additivepolarity. If the secondary winding 38 is wound in subtractive polarity,then the output terminals of the inductor 32 serve as positive andnegative terminals, which are opposite to those of the inductor 32 wherethe secondary winding 38 is additionally wound in additive polarity tothe primary winding 33. The inductor 32 whose secondary winding 38 iswound in subtractive polarity outputs a voltage of V_(AG)×(N1−N2)/N1 tothe load 20 whereas the inductor 32 whose secondary winding 38 is woundin additive polarity outputs a voltage of V_(AG)×(N1+N2)/N1 to the load20. The inductor 32 whose secondary winding 38 is wound in subtractivepolarity is effective for use with a semiconductor switch made of acompound semiconductor or the like and having an ultrahigh withstandvoltage.

A high-voltage pulse generating circuit 10D according to a fourthembodiment of the present invention will be described below withreference to FIGS. 5 through 7. The high-voltage pulse generatingcircuit 10D according to the fourth embodiment, shown in FIG. 5 uses adischarging gap 50 as the load 20.

The high-voltage pulse generating circuit 10D according to the fourthembodiment is substantially the same as the high-voltage pulsegenerating circuit 10B (see FIG. 3) according to the second embodiment,but differs in that it has a diode 52 connected parallel to the firstsemiconductor switch 34. The diode 52 has anode and cathode terminalsconnected to the cathode and anode terminals of the first semiconductorswitch 34, respectively, and hence is in inverse-parallel connection tothe first semiconductor switch 34.

In the embodiment shown in FIG. 5, two parallel-connected diodes 36 a,36 b are used as the diode 36, which is connected between the terminal42 of the inductor 32 and the gate terminal G of the first semiconductorswitch 34. The diodes 36 a, 36 b are functionally identical to the diode36 in the high-voltage pulse generating circuit 10A according to thefirst embodiment.

The high-voltage pulse generating circuit 10D according to the fourthembodiment operates as follows: When the power MOSFET 26 is turned on, acurrent flows through the excited inductance of the inductor 32 asindicated by a path 54 in FIG. 5, storing energy in the inductor 32. Thepolarities of the inductor 32, which are indicated by the black points,are set as shown in FIG. 5. Since the primary winding 33 has a positivepolarity on the side of the black point by a voltage applied thereto,the secondary winding 38 has a positive polarity on the side of theblack point, if the secondary winding 38 is open. For example, as shownin FIG. 2C, when the second semiconductor switch 14 and the firstsemiconductor switch 34 are turned on at time t₀, the first winding 33of the inductor 32 is applied with the substantially same voltage as thedirect power supply voltage E, as described above. Therefore, a negativevoltage is generated across the secondary winding 38 of the inductor 32,the negative voltage having the substantially same value as the directpower supply voltage E×the ratio n between the numbers of turns (see thedashed waveform in FIG. 2C). When the power MOSFET 26 is turned off, thecurrent that has flowed from the anode terminal A to the cathodeterminal K of the first semiconductor switch 34 is commutated from theanode terminal A to the gate terminal G. The electric charges remainingin the first semiconductor switch 34 are drawn from the gate of thefirst semiconductor switch 34, which is then turned off.

When the first semiconductor is turned off, as shown in FIG. 6, thecurrent that has flowed through the excited inductance of the inductor32 is commutated through the inductor 32 to the load 20. At this time, alarge pulse voltage is generated across the inductor 32, producing anelectric discharge across the gap 50 of the load 20.

Since a parasitic capacitive component exists in general semiconductorswitches, including the first semiconductor switch 34, not all thecommutated current flows through the load 20, but some of the currentflows to charge the parasitic capacitance of the first semiconductorswitch 34.

If the load comprises a capacitive load such as the discharging gap 50,then the energy is consumed by an electric discharge. However, not allthe energy may be consumed or no electric discharge may occur, and muchenergy remains stored.

The remaining electric charges are discharged through the excitedinductance of the inductor 32, i.e., a current flows through the excitedinductance of the inductor 32, so that energy is moved again into theexcited inductance of the inductor 32.

When the electric charges stored in the load 20 are gone and themovement of energy into the excited inductance is finished, a currentflow through two paths, i.e., first and second paths 60, 62, as shown inFIG. 7.

The first path 60 is a path directed toward the load 20 again, and thesecond path 62 is a path interconnecting the DC power supply unit 12,the diode 30 arranged in inverse-parallel connection to the power MOSFET26, and the diode 52 arranged in inverse-parallel connection to thefirst semiconductor switch 34.

The voltage generated by the inductor 32 is clamped by voltage generatedby the DC power supply unit 12 and the two diodes 30, 52, and much ofthe current flows through the second path 62. The flow of the currentthrough the second path 62 serves to regenerate energy in the capacitor24 of the DC power supply unit 12 in FIG. 7.

Stated otherwise, excessive energy (unused energy) from the load isreturned to the DC power supply unit 12, contributing to a higherefficiency of operation of the DC power supply unit 12.

If the diode 52 were not employed, then the excited inductance of theinductor 32 and the load 20 would resonate, possibly applying a reversedvoltage in excess of the withstand voltage to the first semiconductorswitch 34. At this time, the second semiconductor switch 14 would beadversely affected, e.g., would be caused to malfunction, by pulsednoise added to the applied voltage. Therefore, it is preferable toconnect the diode 52 for the purpose of processing energy in the excitedinductance.

In the high-voltage pulse generating circuit 10D according to the fourthembodiment, the diode 52 is connected in inverse-parallel connection tothe first semiconductor switch 34. FIG. 8 shows a high-voltage pulsegenerating circuit 10E according to a fifth embodiment of the presentinvention, which has a diode 64 having an anode terminal connected tothe negative terminal 48 of the DC power supply unit 12 and a cathodeterminal connected to a terminal of the inductor 32.

A current flows through a path 66 interconnecting the DC power supplyunit 12 and the diode 64, regenerating energy in the DC power supplyunit 12. The high-voltage pulse generating circuit 10E according to thefifth embodiment is particularly advantageous in that since only onediode, i.e., the diode 64, is connected to the path of the regeneratingcurrent, unlike the above embodiment shown in FIG. 5, any loss causedupon regeneration of energy is small, and the regeneration efficiency isincreased because wiring of the path of the regenerating current can beshortened mechanically.

High-voltage pulse generating circuits 10F, 10G according to sixth andseventh embodiments of the present invention will be described belowwith reference to FIGS. 9 through 12B.

The high-voltage pulse generating circuit 10F according to the sixthembodiment is substantially the same as the high-voltage pulsegenerating circuit 10D (see FIG. 5) according to the fourth embodiment,but differs in that, as shown in FIG. 9, a capacitor 68 is connectedparallel to the first semiconductor switch 34 between the anode andcathode terminals of the first semiconductor switch 34.

The high-voltage pulse generating circuit 10G according to the seventhembodiment is substantially the same as the high-voltage pulsegenerating circuit 10D (see FIG. 5) according to the fourth embodiment,but differs therefrom in that, as shown in FIG. 10, a capacitor 70 isconnected parallel to the first semiconductor switch 34 between theanode and gate terminals of the first semiconductor switch 34.

The high-voltage pulse generating circuits 10F, 10G according to thesixth and seventh embodiments operate as follows: When the power MOSFET26 is turned off, the current that has flowed from the anode terminal tothe cathode terminal of the first semiconductor switch 34 is commutatedfrom the anode terminal to the gate terminal thereof. The electriccharges remaining in the first semiconductor switch 34 are drawn fromthe gate terminal, whereupon the first semiconductor switch 34 isshifted into a turn-off state. At this time, as shown in FIG. 11, acurrent I_(A) which has flowed through the first semiconductor switch 34is commutated to a path 72 along the capacitor 68 (the sixth embodimentshown in FIG. 9) or a path 74 along the capacitor 70 (the seventhembodiment shown in FIG. 10), reducing the operational burden of thefirst semiconductor switch 34.

If the capacitor 68 or 70 were not connected, then, as shown in FIG.12A, the anode current I_(A), which has flowed through the firstsemiconductor switch 34, would be reduced as the power MOSFET 26 isturned off. An indicated by the broken-line curve A, an anode-to-cathodevoltage V_(AK) of the first semiconductor switch 34 would sharply risesubstantially at the same time that the power MOSFET 26 is turned off.The anode-to-cathode voltage V_(AK) would suffer overshooting (pulsedistortion), resulting in an increased switching loss (voltage×current)caused by the first semiconductor switch 34 as indicated by thebroken-line curve C in FIG. 12B. With the capacitor 68 or 70 beingconnected, as indicated by the solid-line curve B in FIG. 12A, theanode-to-cathode voltage V_(AK) rises gradually. Therefore, theswitching loss caused by the first semiconductor switch 34 is reduced,as indicated by the solid-line curve D in FIG. 12B.

Consequently, the connected capacitor 68 or 70 is effective to reducethe switching loss caused by the first semiconductor switch 34 and toincrease the current cutoff resistance of the first semiconductor switch34.

The increased current cutoff resistance leads to an increase in thecapacity of the pulsed power supply. Specifically, since the energystored in the excited inductance of the inductor 32 is determined by½×(the excited inductance)×(the cutoff current of the firstsemiconductor switch 34)², the cutoff current of the first semiconductorswitch 34 greatly affects the output capacity of the power supply.

When the first semiconductor switch 34 cuts off a current at a highspeed or cuts off a large current, a large surge voltage (pulsed output)is applied to the excited inductance of the inductor 32 and the firstsemiconductor switch 34. The surge voltage in excess of the voltagerating would adversely affect the first semiconductor switch 34 whenapplied to the first semiconductor switch 34. However, as describedabove, the connected capacitor 68 or 70 is effective to reduce the surgevoltage for thereby increasing the reliability of the firstsemiconductor switch 34.

According to the type used, the first semiconductor switch 34 may nothave a high voltage rise rate (dv/dt) at the time it is turned off. Thecapacitor 68 or 70 connected parallel to the first semiconductor switch34 is effective to adjust the voltage rise rate (dv/dt) of the firstsemiconductor switch 34 to an allowable level, e.g., 1 kV/μsec, with thecapacitance of the capacitor 68 or 70, thereby increasing number ofdesigns for the high-voltage pulse generating circuits 10F, 10G.

Since much of the energy remaining in the capacitor 68 or 70 thusconnected is regenerated in the DC power supply unit 12, any reductionin the efficiency which is caused by the capacitor 68 or 70 is small.

A high-voltage pulse generating circuit 10H according to an eighthembodiment of the present invention will be described below withreference to FIG. 13.

The high-voltage pulse generating circuit 10H according to the eighthembodiment is substantially the same as the high-voltage pulsegenerating circuit 10D (see FIG. 5) according to the fourth embodiment,but differs in that a capacitor 76 is connected parallel to the load 20.

The high-voltage pulse generating circuit 10H according to the eighthembodiment operates as follows: When the first semiconductor switch 34is turned off, the current that has flowed through the excitedinductance of the inductor 32 is commutated through the inductor 32 tothe load 20. Since the capacitor 76 is connected parallel to the load20, the current that has flowed through the excited inductance is easilycommutated to the load 20 after the first semiconductor switch 34 cutsoff the current. As a result, as with the high-voltage pulse generatingcircuits 10F, 10G according to sixth and seventh embodiments, the firstsemiconductor switch 34 is reduced in size, suffers a reduced switchingloss, and has an increased current cutoff resistance, and the pulsedpower supply has an increased capacity.

When the first semiconductor switch 34 cuts off a current at a highspeed or cuts off a large current, a large surge voltage (pulsed output)is applied to the excited inductance of the inductor 32 and the firstsemiconductor switch 34. However, the capacitor 76 connected parallel tothe load 20 can absorb the energy stored in the excited inductance ofthe inductor 32, thus suppressing the surge voltage on the excitedinductance.

Since much of the energy remaining in the capacitor 76 thus connected isregenerated in the DC power supply unit 12, any reduction in theefficiency which is caused by the capacitor 76 is small.

The capacitor 76 connected parallel to the load 20 greatly affects thepulse duration of the pulsed output and the rise of the pulse voltage.Therefore, the capacitor 76 should have settings selected to match thespecifications of the high-voltage pulse generating circuit 10H.

A high-voltage pulse generating circuit 10I according to a ninthembodiment of the present invention will be described below withreference to FIG. 14.

The high-voltage pulse generating circuit 10I according to the ninthembodiment is substantially the same as the high-voltage pulsegenerating circuit 10A (see FIG. 1) according to the first embodiment,but differs in that a resistor 78, rather than the diode 36, isconnected between the gate terminal G of the first semiconductor switch34 and the terminal 42 of the inductor 32.

When the power MOSFET 26 is turned on, the first semiconductor switch 34can reliably be turned on. If the first semiconductor switch 34 iscontrolled based on current, then it is not turned on unless a currentis introduced into the gate thereof. The resistor 78 connected asdescribed above is effective in reliably turning on the firstsemiconductor switch 34.

Use of the resistor 78 makes the high-voltage pulse generating circuit10I relatively low in cost even if the DC power supply unit 12 isconstructed to produce a high power supply voltage. Specifically, if thediode 36 is connected between the gate terminal G of the firstsemiconductor switch 34 and the terminal 42 of the inductor 32 and theDC power supply unit 12 is constructed to produce a high power supplyvoltage, then the diode 36 needs to comprise a plurality ofseries-connected diodes for an increased withstand voltage or a diodehaving a high withstand voltage, which is generally expensive. Theresistor 78, however, is inexpensive and makes the high-voltage pulsegenerating circuit 10I lower in cost.

The high-voltage pulse generating circuits 10A through 10I according tothe first through ninth embodiments are advantageous over theconventional high-voltage pulse generating circuit 100 and the proposedhigh-voltage pulse generating circuit 118 in that only one firstsemiconductor switch 34 is required as a semiconductor switch to which ahigh voltage is applied, and a gate drive circuit for energizing thegate of the first semiconductor switch 34, is not required.

The circuit components of the high-voltage pulse generating circuits 10Athrough 10I according to the first through ninth embodiments where ahigh voltage is generated or supplied, include only the anode terminal Aof the first semiconductor switch 34 and the terminal 44 of the inductor32. The other circuit components of the high-voltage pulse generatingcircuits 10A through 10I according to the first through ninthembodiments may be circuit components operated with low-voltage.

For example, if the high-voltage pulse generating circuit according tothe present invention is used in an application for decomposingautomobile exhaust gases with a plasma generated by a pulse discharge,then the high-voltage pulse generating circuit may be operated by a DCpower supply having a power supply voltage of about 42 V, which may bean automobile battery, and the circuit components of the high-voltagepulse generating circuit may have a voltage rating up to several dozenV. The conventional high-voltage pulse generating circuit 100 shown inFIG. 15 needs the capacitor charger 102 as a DC power supply, which isusually very expensive.

The high-voltage pulse generating circuits 10A through 10I according tothe first through ninth embodiments can suitably be employed in anapparatus which requires a pulse, having an extremely short rise timeand a high voltage rise rate (dv/dt), such as a plasma generatingapparatus for decomposing toxic gases.

Although certain preferred embodiments of the pre-sent invention havebeen shown and described in detail, it should be understood that variouschanges and modifications may be made therein without departing from thescope of the appended claims.

1. A high-voltage pulse generating circuit for successively outputting apulse of positive polarity and a pulse of negative polarity, comprising:a transformer and a switch which are connected in series across a DCpower supply; wherein an output is produced across a secondary windingof said transformer, wherein a second semiconductor switch is connectedin series between opposite terminals of said DC power supply in additionto said transformer and said switch, an anode terminal of said switch isconnected to a terminal of a primary winding of said transformer, adiode is connected between the other terminal of said primary windingand a gate terminal of said switch, an anode terminal of said diode isconnected to said gate terminal of said switch, and a cathode terminalof said diode is connected to said other terminal of said primarywinding.
 2. A high-voltage pulse generating circuit according to claim1, wherein a first pulse as said pulse of positive polarity or saidpulse of negative polarity is output across said secondary winding ofsaid transformer when said switch is rendered conductive by a turn-on ofsaid second semiconductor switch, and a second pulse having oppositepolarity to said first pulse is output when said switch is turned off bya turn-off of said second semiconductor switch.
 3. A high-voltage pulsegenerating circuit according to claim 1, further comprising a capacitorconnected in parallel to said switch.
 4. A high-voltage pulse generatingcircuit according to claim 1, wherein a capacitive load is connectedacross said secondary winding, further comprising a diode connected inparallel to said switch in a reverse orientation.
 5. A high-voltagepulse generating circuit according to claim 1, wherein said switchcomprises a semiconductor switch.